hnRNP-Proteins

A large set of proteins has
been identified that binds to pre-mRNA.These proteins
have been named hnRNP proteins (hnRNA is a historic
name for RNA in the cell
nucleus, much of which is pre-mRNA). Several
different hnRNP-proteins bind to the same pre-mRNA
immediately as the RNA is transcribed
and an RNA-protein complex is formed.

Many of the hnRNP proteins share the same type of
RNA binding domain.
This concensus RNA binding domain
can also be found in many other proteins that
interact with mRNA, or other kinds of RNA. There are
also other types of RNA binding domains
found in other hnRNP proteins.

The hnRNP proteins presumably
have several different functions, but these are not
well known yet. It has been assumed that hnRNP
proteins help package the pre-mRNA into
functional complexes. For example, hnRNP proteins may
be involved in presenting the pre-mRNA to the
splicing machinery.
It is known that hnRNP A1 protein can influence the
choice of splice sites in pre-mRNA. Three
hnRNP-proteins have been shown to be involved in the
transport of mRNA to the cytoplasm. The hnRNP C
protein contains a retention sequence that appears to
prevent mRNA from being transported out of the
nucleus and hnRNP
A1 and hnRNP K each contain an export signal as part
of their amino acid
chain.

It is further possible that
hnRNP proteins play a role also in the cytoplasm. The
hnRNP A1 protein has been shown to accompany the mRNA
into the cytoplasm and is
present during translation.